Reinforcing Americans' reluctance to engage in entanglements abroad was the suicidal behavior of the European imperialists, who locked themselves in a death struggle starting in 1914. For the first year of the Great War, Americans once again congratulated themselves and their forebears for having left that benighted continent; by consensus the national interest indicated adhering to the advice of Washington and Jefferson about eschewing alliances.
But the twentieth century was not the eighteenth or early nineteenth. American commercial and financial interests had grown tremendously, making the United States more dependent than ever on foreign markets. American merchants demanded the right to trade with the belligerents; American bankers insisted on being able to lend in London and Paris and Berlin. Woodrow Wilson wanted to keep clear of the conflict and tried for a time to rally his compatriots to neutral thinking as well as a neutral policy. But he could not resist the calls for belligerent trade and belligerent loans, and before long the tentacles of capitalism began to draw the United States into the war. Because the British had a better fleet than the Germans, most American trade, and the financing that funded the trade, flowed to Britain and France, making the United States a de facto member of the Allied powers by 1916. Germany recognized the situation, and in early 1917 declared war on American shipping. Wilson responded with a request for a formal war declaration, which Congress granted.
At the time, few in Congress or outside it questioned that the American national interest required defending American vessels and American nationals against German attack. Some, however, did question whether U.S. participation in the war required putting U.S. troops on the ground in Europe. "Good Lord! You're not going to send soldiers over there, are you?" queried a shocked senator of a War Department spokesman. The War Department did send the troops, which helped break the back of Germany's desperate final offensive.
But what else they accomplished was open to doubt. The British and French knew what they were fighting for (although the rest of the world did not until Lenin leaked the secret treaties that spelled out the Allies' imperialistic war aims). The Americans were far less sure of their own purposes, not least because Wilson had waffled all over the landscape of politics and diplomacy. At various times he talked of being too proud to fight, then of achieving peace without victory, then of making the world safe for democracy. His Fourteen Points seemed rather many, with some too vague and others too specific. In any case, the British and French demonstrated at the postwar peace conference that they would have nothing to do with Wilson's airy abstractions. Germany had lost, and Germany would pay—in treasure, territory, colonies, and markets. Wilson could have his precious League of Nations, the proto–world government he hoped would prevent another such war. But he would have to make of it what he could.
Which turned out to be nothing at all. Even more than Wilson, Americans were confused as to why they entered the war. The only national interest that seemed directly threatened was the right to trade with belligerents. But the British violated America's neutral rights as consistently, if less egregiously, than the Germans (who, short on surface ships, had to resort to U-boat torpedoes to enforce a blockade). Was there an American national interest in preventing another European war? On this point, as on any other meaningful topic touching the national interest, the answer turned on the cost. Of course, it would be to America's benefit for Europe to remain at peace, but would the benefit outweigh the cost of an indefinite commitment to enforce the mandates of this new League of Nations? Did Americans wish to play policemen to the world?
The Senate said no in twice rejecting the Treaty of Versailles. Wilson summoned a simple majority for the treaty, but not the two-thirds supermajority ratification required. Under the rules specified by the Constitution, the Senate declared that membership in the League of Nations was not in the American national interest. Whether it really was not in the national interest, time would tell.